1. Field of the Invention
This invention relates to improved joints for connecting carbon electrodes to provide electrode columns for use in electric furnaces or like equipment, the term "carbon" as used herein being generally inclusive of the graphite form of carbon. Such joints permit continuous electrical operation of an apparatus utilizing electrodes by providing means for attaching new electrode material to the end of an electrode column as the column is consumed.
Conventionally, an electrode joint comprises two coaxially aligned carbon electrode sections having threaded sockets in their butt faces and a correspondingly threaded nipple joining the electrode sections in abutting relationship. An alternative to this joint design comprises a "male-female" type electrode joint comprising two electrode sections, each section having a threaded nipple machined directly on one end thereof and a correspondingly threaded socket machined in the other end thereof, the sections being joined by uniting the nipple of one electrode section and the socket of another section.
One serious problem encountered with conventional electrode joints is that they are susceptible to failure during use, this failure resulting from breakage due to splits and cracks produced by stresses caused by flexural strain and thermal gradients experienced by the joint when it is being fed into an operating electric furnace. The difference between the coefficient of thermal expansion of the electrode sockets and the nipple also lead to stresses within the electrode joint. Further, the high current loading of modern electrode furnaces frequently causes additional joint stresses due to overheating of the nipple portion at the junction between the nipple and the electrode sockets.
It is known that failures of electrode joints, which result from the stresses and strains discussed above, are principally localized at the first engaged socket thread in the base of the socket or the first engaged nipple thread near midlength of the nipple for a conventional joint, and near the base of the female member in a "male-female" joint.
2. Description of the Prior Art
It has long been recognized that a modification of the conventional electrode joint design was necessary to eliminate failure due to breakage during use. Various methods have been proposed by those skilled in the art to prevent this failure by modification of the joint at or near the base of the electrode socket.
U.S. Pat. No. 3,708,601--Kozak relates to an electrode joint comprising two electrode sections having threaded sockets designed such that when joined by a nipple, all the threads at the base of the sockets are engaged with mating threads of the nipple. This reference, which provides some reduction of the stresses at the area of the electrode joint at the base of the socket by removing all threads which would not be engaged by the nipple, thereby increasing the thread root angle, does not provide means for distributing the stresses amongst the engaged threads of the joint.
The present invention provides means whereby the stresses at the first few bottom threads of an electrode socket are reduced, thus providing an electrode joint of increased strength, by chamfering the first few threads of each end of the nipple, or the first few bottom threads of each of the sockets. In the case of a "male-female" type carbon electrode joint, previously characterized, the strength of the joint can be substantially increased by chamfering the first few threads at the end of the nipple or the first few bottom threads of the socket which are united to form the electrode joint.